Certain special applications, such as medical devices, machine control, and others, may require large size pixels (in the order of larger than 100 μm pitch) to catch the light beam(s). In additional, these require some kind of circuits in addition to a photon sensing device. These may use, for example, a 3T photodiode (PD), 3T partial pinned photodiode (PPPD), 4T pinned photodiode (PPD), and other structure to try and attain as close as possible to 100% fill factor to collect every photon from the light source.
FIGS. 1A, 1B and 1C show a typical pixel design with a photodiode. FIG. 1A shows the design where the Larger area 100 is the photodiode, and the photodiode 100 is connected to a sensing node 110. The sensing node 110 connected to the auxiliary circuit 111 that can include for example the reset gate, and an output buffer such as a source follower.
FIG. 1B shows an example of doping profile across the cross-section AA′ in FIG. 1A. This doping profile assumes the photodiode structure is an n-type pinned photodiode. FIG. 1C shows the potential diagram at cross-section AA′ of the doping profile in FIG. 1B. For an n-type pinned photodiode, the signal charge is photoelectrons generated by the light.
When using a large size photodiode, the charge transfer from photodiode to the output sensing node becomes a limit to the pixel performance. As shown in FIG. 1C, the potential under the most of the large photodiode region is flat, so there is no electrical field in this region. Photon generated charges (photo-electrons or photo-holes) in this zero electrical field region will diffuse to the sensing node 110, instead of being pushed by any electrical field. The charge diffusion causes random movements in all directions, and hence can be very slow. This charge diffusion time, Tdiff is inversely proportional to the square of the distance of flat potential region, dm. I.e., Tdiff ∝1/(dm)2.
When attempting to carry out any kind of fast readout, the slow movement of the photogenerated charges causes image lag. This image lag could be very high comparing to the signal level.
FIGS. 2A-2C show a prior art solution which uses multiple implants in a single photodiode. One of the common solutions (same as in early works on CCD) is to have multiple implant regions 210, 220, 230, 240 in the photodiode 200, as shown in FIG. 2A. The multiple implant doping profile is shown in FIG. 2B. The multiple implants in the photodiode create a stair type potential in the photodiode region, as shown in FIG. 2C. This stair potential generates an electrical field to push the photon-generated charge to the sensing node 250. The distance of flat potential region, dm, in this multiple implant structure becomes much shorter.
However, using multiple photodiode implants has disadvantages in CMOS sensor applications. Specifically, use of multiple photodiode implants brings complexities into the fabrication process and can thus increase the fabrication costs. And, such a process is not available in many of the image sensor fabrication foundries. In addition, these multiple implants can reduce the pixel output dynamic range. The power supply voltage range to the pixel is a limit to a dynamic range in a CMOS sensor. Each additional implant in the photodiode region causes a drop of the photodiode pinning voltage. Thus, the voltage swing of the sensing node drops significantly if the photodiode has multiple implants. I.e., compare to FIG. 1C, the pixel output swing, VOUT_max in FIG. 2C, becomes much lower. As the results, the pixel output swing as well as the dynamic region becomes very small for a pixel with multiple implants.
Another solution is to split the pixel into multiple pixels. FIG. 3 shows an example to split the photodiode to four (4) photodiodes 300, 301, 302, 303. Each photodiode has its own readout such as 305. However, the separation region 310 between the photodiodes, which is in the middle of the light spot, will possibly cause significant signal loss.